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<h2>GroupByKey</h2>
<p>
  GroupByKey is a Beam transform for processing collections of key/value pairs. It’s a parallel
  reduction operation, analogous to the Shuffle phase of a Map/Shuffle/Reduce-style algorithm. The
  input to GroupByKey is a collection of key/value pairs that represents a multimap, where the
  collection contains multiple pairs that have the same key, but different values. Given such a
  collection, you use GroupByKey to collect all of the values associated with each unique key.
</p>
<p>
  <b>Kata:</b> Implement a
  <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/GroupByKey.html">
    GroupByKey</a> transform that groups words by its first letter.
</p>
<br>
<div class="hint">
  Refer to <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/values/KV.html">
  KV</a> and
  <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/GroupByKey.html">
    GroupByKey</a> to solve this problem.
</div>
<div class="hint">
  Refer to the Beam Programming Guide
  <a href="https://beam.apache.org/documentation/programming-guide/#groupbykey">
    "GroupByKey"</a> section for more information.
</div>
</html>
